Particulate reinforced Al-based composites offer higher specific stiffness and specific strength, good wear-resistance, better fatigue durability, lower thermal expansion coefficient and good dimensional stability, as compared to conventional Al alloys. In addition, the formulas of the composites can be designed in a wide range to meet specific properties, while the conventional Al alloys do not have such capabilities of designability. Therefore, many countries have spent substantial amounts of investments to develop this type of composites, and some of them have been successfully applied in aerospace, military and civilian industries. With the increasing demand for particulate reinforced Al-based composites, fabrication of high performance and cost effective composite components are the focus of current research and development activities. High performance refers to higher mechanical and physical properties with good machinability; while cost effective means to minimize the cost of the composite billets and their final component-forming process cost, especially for the complex components.
At present, four major fabricating processes are used to make particulate reinforced Al-based composite billets, including powder metallurgy (PM), agitation casting, spray forming and squeeze casting. Two key issues to be resolved in the current composite billet fabrication processes are: 1) improvement of the uniformity of the reinforced particle distribution and 2) enhancing their bonding strength in the Al matrix.
It is well-documented that composite properties are controlled by the following key parameters, such as reinforced particle size, their uniformity of distribution, and their interfacial bonding with the matrix. Also, the composite machinability strongly depends on the particle size. Small reinforced particle composites have better machinability. With a view to fabricating high performance and cost effective composites, the desirable parameters include small reinforced particles and uniform distribution, and good interfacial bonding in the matrix. Among the four fabricating processes indicated above, the powder metallurgy (PM) process represents the best one to meet the above parameters. Nevertheless, due to a large particle-size ratio (≧11–28) between the raw Al powder particles (40–100 μm) and the reinforced particles (≦3.5 μm), it is difficult to achieve a uniform distribution of the reinforced particles in the matrix using the conventional mechanical mixing processes. In addition, the surface oxide layer of the Al powder will deteriorate the interfacial bonding strength with the matrix. Thus, high-quality and easy machining composites are hardly fabricated through the ordinary mechanical mixing processes.
U.S. Pat. No. 3,591,362 by Benjamin et. al. provides a theoretical approach to solve this problem. Using a high-energy ball-milling technique, the Al alloy matrix powder particles are deformed repeatedly under grinding and impact by high-energy balls, and a cold-weld layer forms on the ball surface. This cold-weld layer will fall off and be crushed by the continuous work-hardening. Finally, fine composite powers are obtained. Later, U.S. Pat. No. 3,740,210 invented a raw material for the dispersion-strengthened Al composite, consisting of Al powder and its oxide powder. In this process, the raw powders with surfactant are dry-grounded. However, the properties of the composite billet made are deteriorated because the fine composite powders contain the surfactant. Another US patent (U.S. Pat. No. 4,946,500) introduced a method to fabricate Al-based composites, consisting of Al-alloy powder and reinforced particle powder. The raw powders are mixed under a high-energy ball-milling process without adding any surfactant, thus eliminating the negative effect on the properties of the final composite billet. However, cold-welding tends to be more severe during ball-milling without adding surfactant, resulting in an unstable mixing/homogenizing process. Thus, it is not suitable for continuous industrial production. This patent does not specify how to solve the cold-weld problem in case no surfactant is added. In addition, the particle size of the ball-grinding composite powder is too large when surfactant is absent during the milling process. As a result, the billet produced cannot meet requirements in the subsequent compressing forming process.
After acquiring the high performance composite billet fabrication technology, the next key issue is how to reduce the process cost, especially for the complex shape composite components. The near net shape approach is the most cost effective way for making composite components. Machining and mold-forging are the commonly used fabricating methods for components. However, machining would increase the cost due to the composite's poor machinability. Cost of mold forging is also higher because of the composite's poor plastic deformation characteristics.
Semisolid forming components is one of the near net shape forming processes. Thanks to the fine particle size of either the Al matrix powder or the reinforced particles, the composite billet should have a thixotropic characteristic for semisolid processing. A semisolid near net shape forming composite approach has been proven successful using a spray forming composite billet as described in a recent US patent (U.S. Pat. No. 6,135,195). This patent invented a thixotropic composite of SiC/2xxxAl, the composite billet being made by a spray forming process. To ensure the thixotropy of the composite billet, additional Si of 1–5 wt % is added into the standard Al alloy. Also, a well-controlled double heating method is used. However, this patent does not state whether or not this material has its thixotropic nature without adding more Si. Normally, the composite billet prepared by the spray forming process exhibits poor macroscopic distribution of the reinforced particles, and the composite properties are, therefore, not consistent.